Miniaturized piezoelectric accelerometers are in high demand for applications in aerospace, automobiles, military systems, and machine condition monitoring. Various acceleration sensing mechanisms have been used in accelerometers, including electrostatic, capacitive, piezoresistive, and piezoelectric. Compared to other accelerometers, piezoelectric accelerometers have significant advantages of quick response, high performance at high frequency, high output impedance, low power consumption, and the potential of being self-powered.
Micro electromechanical systems (MEMS) has been applied to produce miniaturized piezoelectric accelerometers using piezoelectric thin films, with the objectives of further miniaturization, improved production consistency, reduced unit cost and the capability of integrating multiple functions. However, the small thickness of piezoelectric thin films significantly restricts the magnitude of electrical voltage output from miniaturized piezoelectric MEMS accelerometers. Therefore, efforts have been made in the prior art to design and fabricate miniaturized piezoelectric MEMS accelerometers by aligning the electrical polarization with the surface plane of the piezoelectric thin films. In such an in-plane design, a pair of piezoelectric electrodes is deposited on the top of the piezoelectric thin films to replace the conventional sandwich electrode configuration. In such an arrangement: (i) the limitation of the small thickness on the electrical output voltage of the piezoelectric accelerometers is compensated and the voltage sensitivity is improved, (ii) the electrical output generated over the electrodes is determined by the longitudinal piezoelectric coefficient (commonly referred to as d33), which is usually significantly larger than the transverse piezoelectric coefficient (commonly referred to as d31) as utilized in the sandwich electrode configuration, and (iii) the relevant fabrication and packing process become simpler without the need to use and connect any bottom electrode.
Conventional piezoelectric MEMS design with inter-digital electrodes includes a micro cantilever structure and a diaphragm structure. Piezoelectric micro-cantilever structures, which comprise multiple thin layers including a piezoelectric ceramic thin film, often deform seriously due to large residual stress. They are very brittle and liable to break under a large mechanical shock. Piezoelectric diaphragm structures are much more robust but the structure is very rigid, particularly when the size of the diaphragm is further reduced, which in turn unfavourably limits the sensitivity at small sizes.